Aluminum metal is conventional produced by the electrolytic reduction of alumina dissolved in a molten cryolite bath according to the Hall-Heroult process.
This process for reducing alumina is carried out in a thermally insulated cell or "pot" which contains the alumina-cryolite bath. The cell floor, typically made of a carbonaceous material, overlies some of the thermal insulation for the cell and serves as a part of the cathode. The cell floor may be made up of a number of carbonaceous blocks bonded together with a carbonaceous cement, or it may be formed using a rammed mixture of finely ground carbonaceous material and pitch. The anode, which usually comprises one or more carbonaceous blocks, is suspended above the cell floor. Resting on the cell floor is a layer or "pad" of molten aluminum which the bath sees as the true cathode. The anode, which projects down into the bath, is normally spaced from the pad at a distance of about 1.5 to 3.0 inches (3.81 to 7.61 cm). The alumina-cryolite bath is maintained on top of the pad at a depth of about 6.0 to 12.0 inches (15.24 to 30.48 cm).
As the bath is traversed by electric current, alumina is reduced to aluminum at the cathode and carbon is oxidized to its dioxide at the anode. The aluminum thus reduced is deposited on the pad and tapped off periodically after it has accumulated.
For the electrolytic process to proceed efficiently, the alumina reduction should occur onto a cathode surface of aluminum and not the bare carbonaceous surface of the cell floor. Therefore, it is considered important for the pad to cover the cell floor completely.
As molten aluminum does not readily wet or spread thinly on carbonaceous materials, the pad can best be visualized as a massive globule on the cell floor. In larger cells, the dense currents of electrolysis give rise to powerful magnetic fields, sometimes causing the pad to be violently stirred and to be piled up in selected areas within the cell. Therefore, the pad must be thick enough so that its movements do not expose the bare surface of the cell floor. Additionally, the anode must be sufficiently spaced from the pad to avoid short circuiting and to minimize reoxidation of aluminum.
Still, the movements of the pad have adverse effects which cannot always be readily controlled. For a given cell operating with a particular current of electrolysis, there is an ideal working distance between the cathode and the anode for which the process will be most energy efficient. However, the required spacing of the anode due to turbulence of the pad prevents this ideal working distance from being constantly maintained. Further, since the pad is in a state of movement, a variable, nonuniform working distance is presented. This variable interelectrode distance can cause uneven wear or consumption of the anode. Pad turbulence can also cause an increase in back reaction or reoxidation at the anode of cathodic products, which lowers cell efficiency. In addition, pad turbulence leads to accelerated bottom liner distortion and degradation through thermal effects and through penetration by the cryolite and its constituents. It has been suggested in the literature and prior patents that certain special materials, such as refractory hard metals (RHM), most notably titanium diboride (TiB.sub.2) or its homologs, can be used advantageously in forming the cell floor. Further, it has been found that RHM shapes may be imbedded into or placed onto the cell floor, rising vertically through the molten aluminum layer and into the cyrolite-alumina bath, with the uppermost ends of these shapes forming the true cathode. When such a cathode design is employed, precise spacing between the true or active surfaces of the cathode and the anode may be maintained, since such a system is not effected by the ever-moving molten aluminum pad acting as the true cathode surface.
Ideally, in contrast to conventional carbon products, these RHM materials are chemically compatible with the electrolytic bath at the high temperatures of cell operation and are also comparable chemically with molten aluminum.
Furthermore, these special cell floor materials are wetted by molten aluminum. Accordingly, the usual thick metal pad should no longer be required, and molten aluminum may be maintained on the cell floor as a relatively thin layer and commensurate with amounts accumulating between the normal tapping schedule.
With all their benefits, there is a problem associated with the use of RHM shapes in alumina reduction cells. These shapes are extremely brittle, and may be broken by contact with an anode lowered thereupon. Anode movement in a cell occurs quite often during aluminum production, due to the need to change anodes, tap aluminum from the cell or adjust the voltage within the cell. Should these shapes be accidently contacted by a lowered anode, and thus broken, increased down time results, due to the need to again raise the anode and replace the shapes, or, in a more extreme case, drain the cell, replace the shapes, and restart the cell.
In U.S. Pat. No. 4,436,598, the disclosure of which is hereby incorporated herein by reference, it is suggested to position anode stops within the cell. These stops are imbedded within the carbonaceous cathode and extend into the alumina-cryolite bath for a greater distance than the RHM shapes. If the anode is lowered upon these stops, the stops protect the RHM shapes from contact and breakage. The stops are formed of a material which is not a conductor of electricity, so that the RHM shapes remain the true cathode. Suitable materials for the anode stops include silicon nitride, silicon carbide, aluminum nitride and boron nitride.
It has been found, however, that, while these anodes stops were effective in protecting the RHM shapes, and while the portion of the anode stop within the aluminum pad exhibits very little deterioration during cell operation, the portion of the anode stops projecting into the alumina-cryolite bath eventually eroded away due to the solubility of the refractory material in the cryolite. Thus, it has been found that these anode stops loose their effectiveness in protecting the RHM shapes after about six months of operation in the cell. Because the anode stops were permanently mounted within the cell, replacement of these stops required shutdown and drain of the cell, which is not cost effective.
There remains, however, a need for effective protection of refractory hard metal shapes in alumina reduction cells. It is thus a primary objective of the present invention to provide an improved protection system for refractory hard metal shapes in alumina reduction cells.